But it is a Big mod. I can make a mini mod for you with only the new UI textures if you want to try and see if it helps. Using some tricks it has a higher resolution than vanilla textures and icons.. Does not help for the text font in the other game menu's, but I am working on a fix for that too. I can give you a quickfix I got from Brother UnsocialLobster if you want to try that too. The Soulstorm Turbo Mod (moddb) has hacked grapicsconfig that allows you to select more screen resolutions. those 3 combined should give you what you seek.

To my knowing there is no mod and probably never will be. The game UI wasn't designed for 16:9 or other aspect ratios.

I just launched vanilla in 1920x1080p and didn't see any ui stretching.

Do it the other way around. start in 4:3 aspect ratio (like 1024x768) as the game was original meant to be played (~2004-2005) and then you see how it should look.

At least if you switch later to 1920x1080p again, you see the difference

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adjusting all the vanilla UI textures in aspect to fit the 16:9 ratio is an option, but that's not what I would want to do. Oh, and that must be done for all race mods as well. The results could look worse then the stretching we see now I can imagine.